IRS-Assisted Covert Communication with a BPP Distributed Warden outside a Safety Zone

Zhilin Chen, Shihao Yan, Xiaobo Zhou, Feng Shu, Jiande Sun, Derrick Wing Kwan Ng

Abstract: In this work, we consider an intelligent reflecting surface (IRS)-assisted covert wireless communication system, in which the location of a warden Willie follows a binomial point process (BPP) distribution within a disk centered at O with a radius of R and the warden is outside a safety zone centered at C with a radius of r. The location of the safety zone can be anywhere (inside, outside, or having intersections) relative to the disk. We first analyze the detection performance of Willie for the cases with known channel state information (CSI) and unknown CSI, respectively, based on which we determine Willie's minimum detection error rate. Then, considering the geometric randomness of Willie's position inside the disk but outside the safety zone, the average minimum detection error rate is determined and adopted in the covertness constraint in the subsequent covert system design. Our examination first shows that the covert communication rate generally increases with R. However, more interestingly, whether the covert communication rate increases or decreases with r generally depends on the relative locations of the disk, the safety zone, and Alice. For example, when both the safety zone and Alice are on the same side of the disk center O, the covert rate increases with r, and vice versa.